Recently, miniaturization in an electronic component to be mounted has been rapidly advanced according to development in small information equipment. In order to cope with narrowing of a connection terminal or reduction of a mounting area according to a demand of the miniaturization, a ball grid array (BGA) in which electrodes are disposed on a rear surface thereof has been applied to the electronic component.
The electronic component, to which the BGA is applied, for example, is a semiconductor package. The semiconductor package is configured by sealing a semiconductor chip including electrodes with resin. A solder bump is formed on each of the electrodes of the semiconductor chip. The solder bump is formed by allowing a solder ball to adhere to the electrode of the semiconductor chip. The semiconductor package, to which the BGA is applied, is mounted on a printed circuit board by allowing the solder bump fused by heating to adhere to a conductive land of the printed circuit board. Recently, in order to further cope with a demand of high density packaging, three-dimensional high density packaging in which the semiconductor packages are stacked in a height direction, has also been developed.
The semiconductor package, to which the three-dimensional high density packaging is applied, is a BGA. When the solder ball is mounted on each of the electrodes of the semiconductor chip and a reflow process is performed thereon, there may be a case where the solder ball is crushed due to the weight of the semiconductor package itself. In such a case, the solder may protrude from the electrode, and the electrodes are in contact with each other, and thus, there may be a concern that a short circuit between the electrodes occurs.
In order to prevent such a short circuit, a solder bump has been proposed in which a solder ball is not crushed by the weight of the semiconductor package itself or is not deformed at the solder melting time. Specifically, it has been proposed that a core material in which a ball molded with metal or resin is used as a core and the core is covered with a solder, is used as a solder bump.
A lead-free solder containing Sn as a main ingredient has been often used as a solder plating layer covering the core. An Sn-based solder alloy made of Sn and Bi is exemplified as a preferred example (see Japanese Patent Application Publication No. 2007-44718 and Japanese Patent No. 5367924).
In a core material disclosed in Japanese Patent Application Publication No. 2007-44718, a Cu ball is used as a metal, the Cu ball is used as a core, and an Sn-based solder alloy made of Sn and Bi is formed on the surface of the core as a solder plating layer. Since the Sn-based solder alloy containing Bi has a comparatively low melting point of 130° C. to 140° C., it is used as a plating composition from a reason that a thermal stress on a semiconductor package is small.
In Japanese Patent Application Publication No. 2007-44718, a plating treatment is performed in a concentration gradient where the content of Bi contained in the solder plating layer decreases as being directed towards an inner side (an inner circumference side) and increases as being directed towards an outer side (an outer circumference side).
From the same reason as that of Japanese Patent Application Publication No. 2007-44718, Japanese Patent No. 5367924 discloses a solder bump in which a Cu ball is used as a core, and the core is plated with an Sn-based solder alloy made of Sn and Bi. In Japanese Patent No. 5367924, a plating treatment is performed in a concentration gradient where the content of Bi contained in a solder plating layer increases as being directed towards an inner side (an inner circumference side) and decreases as being directed towards an outer side (an outer circumference side).
In a technology of Japanese Patent No. 5367924, the concentration gradient is completely opposite to that of Japanese Patent Application Publication No. 2007-44718. It is considered that this is because concentration control in Japanese Patent No. 5367924 is simpler than that of Japanese Patent Application Publication No. 2007-44718, and thus, manufacturing thereof is easily performed.